Alkyllithiums acetylenic

Although the chemistry of anionic cyclizations of organolithium moieties derived from acetylenic systems has been less investigated38, studies of some acetylenic alkyllithiums, e.g. 131 (formed from 130), have shown that the ring closure proceeds in a regiospecific and highly stereoselective syw-fashion to give exocyclic vinyllithiums 132 (Scheme 43). 

In light of the facile 5-exo-dig carbolithiation reaction of simple acetylenic alkyllithiums, Bailey and Longstaff have studied the analogous 5-exo cyclization of a benzyne-tethered alkyllithium. Regioselectively 4-functionalized indanes 281 have been prepared... 

The conversion of acetylenes into acetyl ides, M-C=C-R (M = Li, Na, K, MgBr), by means of alkyllithium or Grignard reagents in organic solvents or by alkali metal amides in liquid ammonia is well documented (for practical examples see ref. 1, for review articles consult inter alia refs. 2-5). 

In contrast to the metallation with alkyllithium or alkali amides, Grignardation of HOCR with alkylmagnesium halide is not an instantaneous reaction. The more acidic 1,3-diynes and hetero-subsdtuled acetylenes react most readily. In the other cases heating for several minutes... 

The procedure described here is characterized by good yields, mild conditions, and easy synthesis of a pure form from readily available starting materials. Since tertiary aliphatic acetylenes do not form readily under these conditions, the excess of alkyllithium used is not particularly critical. The small amount of by-products that also form is similarly readily removed at the distillation stage. 

Metallation of alkynylcyclopropanes at the acetylenic end is accomplished either by deprotonation or via metal-halide exchange reaction with strong bases. Metallation of ethynylcyclopropane may be affected by KOH in DMF, ethereal EtMgBr or preferably BuLi in THF (equation 151)231. All three metal acetyl ides react with methyl ketones to give the corresponding alcohols. However, the instability of cyclopropyl ketones towards bases, especially at the reaction conditions required by KOH (20 °C, 6h), and the sensitivity of cyclopropenyl double bonds in cyclopropenyl ketone derivatives towards addition reactions of alkylmagnesium compounds, make the alkyllithium (-78 °C, instant reaction) superior to the other reagents. 

A subtle, but important, point must be made before we can extend our understanding of acid-base chemistry to the reaction between a Grignard or alkyllithium reagent and a carbonyl group. The data in the table of Br /nsted acids and their conjugate bases reflect the strengths of common acids and bases when they act as Brif/nstedacids or bases. These data predict that methyllithium should react with acetylene to form methane and an acetylide ion, for example. 

The addition of amines and ethers to alkyllithium compounds profoundly affects polymerization of such species. Amines and ethers alter the association of RLi compounds and change the course of the polymerization and its kinetics. Also, the presence of small amounts of such impurities as water, alcohols, or a-acetylenes, influences the kinetic chain length. The chain-termination reaction with such acidic protons is almost instantaneous. However, there are certain types of protons, such as a-aromatic, secondary amine, and /3-acetylenic, that are not acidic enough to react immediately but will undergo transmetalation during the course of a polymerization reaction. This results in termination or chain transfer of the polymer chain, and limits the realization of polymers of... 

Organolithium compounds RLi (R = Me, Et, //-Bu, Ph) are excellent dehydrohalo-genating agents of vinyl halides in ether or THE solution under very mild conditions at temperatures below 0 °C. In a first step the acidic geminal hydrogen is replaced by lithium in a slow step to give an alkenyllithium which can be isolated at low temperature. In a second fast step lithium halide is eliminated to furnish an acetylene (equation 61). Two equivalents of alkyllithium are needed according to this equation. 

A filled sp orbital is lower in energy than filled sp or sp orbitals since it is closer to the positively charged nucleus. This imparts sufficiently greater acidity to acetylene and 1-alkynes (pA a 24-26) so that bases such as alkyllithiums, lithium dialkylamides, sodium amide in liquid ammonia, and ethylmagnesium bromide may be used to generate the alkynyl anions (see Section 8.2). 

The most utilized method for alkylation of alkynes is via alkynylide anions. Their reaction with electrophilic reagents provides access to both terminal and internal alkynes as well as to functionally substituted alkynes. The higher electronegativity of carbon in the sp-hybridization state imparts relatively greater acidity to acetylene and 1-alkynes (pATa 24-26), so that bases such as alkyllithiums, lithium dialkylamides. 

Polymetalation is apparently a general reaction of 1-alkynes. Lithia-tion of 1-butyne has been studied in detail (2). When this compound is treated with three equivalents of n-butyllithium or terf-butyllithium in hydrocarbon solvents, trilithio derivative, CH3C3Li3, is formed. Noncon-jugated internal acetylenes resist metalation by alkyllithium reagents... 

---